1. Field of the Invention
The present invention relates to a piezo-resistance type triaxial acceleration sensor used for portable terminal equipment and other similar small devices for detecting acceleration in three axis directions (X, Y and Z axis directions), and more particularly to a technology for reducing the output difference among the three axes (X, Y and Z axes) so as to save power.
2. Description of the Related Art
Conventional technology on an acceleration sensor for decreasing the output difference among the three axes (X, Y and Z axes) is, for example, disclosed in Japanese Patent Application Kokai (Application Laid-Open) No. 2003-279592.
FIG. 4A and FIG. 4B of the accompanying drawings depict the conventional piezo-resistance type triaxial acceleration sensor disclosed in Japanese Patent Application Kokai No. 2003-279592. FIG. 4A is a plan view and FIG. 4B is a cross-sectional view taken along the line 4B-4B in FIG. 4A.
This acceleration sensor is made from a silicon mono-crystal substrate, and has a square frame section 1. At each of the four corners inside the frame section 1, an opening 2 penetrates through in the Z axis direction, that is the vertical direction to the FIG. 4A drawing sheet, and at the inner center of these four openings 2, a mass section 3, which is a thick weight, is disposed. The mass section 3 is flexibly connected to the frame section 1 by the thin beam section 4. The beam section 4 includes two X-direction beam elements 4-1a and 4-1b extending in the X axis direction, which is the width direction of the FIG. 4A drawing sheet, and two Y-direction beam elements 4-2a and 4-2b extending in the Y axis direction, which is in the height direction of the FIG. 4A drawing sheet. These four beam elements 4-1a, 4-1b, 4-2a and 4-2b supports the mass section 3 at the center of the acceleration sensor such that the mass section 3 can move to a certain extent in the X, Y and Z directions.
On the X-direction beam elements 4-1a and 4-1b, two pairs of X-axis piezo-resistors 5-1a and 5-1b and 5-2a and 5-2b are formed, and also two pairs of Z-axis piezo-resistors 5-5a and 5-5b and 5-6a and 5-6b are formed. Likewise, on the Y-direction beam elements 4-2a and 4-2b, two pairs of Y-axis piezo-resistors 5-3a and 5-3b and 5-4a and 5-4b are formed. Each piezo-resistor 5-1a and 5-1b to 5-6a and 5-6b has the same shape (same length) with the same resistance value.
The acceleration detection principle is briefly described here. The deflections of the beam elements 4-1a, 4-1b, 4-2a and 4-2b, when the mass section 3 displaces upon receiving a force in proportion to the acceleration, are detected as the resistance change of the piezo-resistor pairs formed on the beam elements 5-1a and 5-1b, 5-2a and 5-2b, 5-3a and 5-3b, 5-4a and 5-4b, 5-5a and 5-5b and 5-6a and 5-6b, so as to detect the acceleration in the three axis directions. The two pairs of the piezo-resistors 5-1a and 5-1b and 5-2a and 5-2b on the beam elements 4-1a and 4-1b detect acceleration in the X axis direction, the two pairs of the piezo-resistors 5-5a and 5-5b and 5-6a and 5-6b on the beam elements 4-1a and 4-1b detect the acceleration in the Z axis direction, and the two pairs of the piezo-resistors 5-3a and 5-3 b and 5-4a and 5-4b on the beam elements 4-2a and 4-2b detect the acceleration in the Y axis direction. For such acceleration detection, each four piezo-resistors (two pairs of piezo-resistors) of each axis are wired to construct a bridge circuit independently.
The relationship between the positions of the piezo-resistors and the sensor output in each X, Y and Z axis will now be described. As an example, the X-axis piezo-resistors 5-1a and 5-1b and the Z-axis piezo-resistors 5-5a and 5-5b formed on the beam element 4-1a are used.
Although this is different from FIG. 4A, it is assumed that the X-axis piezo-resistor 5-1a and the Z-axis piezo-resistor 5-5a are positioned contacting the boundary line P1 with the frame section 1, and the X-axis piezo-resistor 5-1b and the Z-axis piezo-resistor 5-5b are positioned contacting the boundary-line P2 with the mass section 3. In this case, stress concentrates on the area around the frame section 1 and the mass section 3 in this beam element 4-1a when the beam element 4-1a deflects by the acceleration received, so the maximum sensor output can be obtained. The sensitivity characteristic (output with respect to angular velocity 1 G and drive voltage 1V) differs between the X axis and the Y axis. The sensitivity in the X axis changes quadratic-functionally, and the sensitivity in the Z axis changes linear-functionally.
In other words, if an acceleration of 1 G is applied in the X axis direction, the bending moment to be applied to the beam element 4-1a is given by the product (s1×m) of the distance s1, from the plane which passes through the beam elements 4-1a, 4-1b, 4-2a and 4-2b to the center of gravity of the mass section 3, and the mass m of the mass section 3. Therefore if the thickness of the mass section 3 changes, the bending moment is in proportion to s1 and m and the sensitivity of the X axis changes quadratic-functionally. On the other hand, if an acceleration of 1 G is applied in the Z axis direction, the bending moment to be applied to the beam element 4-1a is given by the product (s2×m) of the length s2 of the beam element 4-1a and the mass m of the mass section 3. Therefore if the thickness of the mass section 3 changes, the bending moment is in proportion only to m, and the sensitivity of the Z axis changes linear-functionally.
To eliminate the output difference between the X axis and the Y axis, the thickness of the mass section 3 should be set to a value at the intersection of the quadratic function curve, which indicates the sensitivity of the X axis, and the linear function curve, which indicates the sensitivity of the Y axis (e.g. about 800 μm). However the thickness of the silicon mono-crystal substrate used for semiconductors is mostly 625 μm or 525 μm, so a silicon mono-crystal substrate with about an 800 μm thickness requires special ordering, which increases cost. In other words it is not preferable to adjust the output by the thickness of the mass section 3.
If a silicon mono-crystal substrate with a 625 μm or 525 μm thickness, which can be easily acquired, is used, the linear function curve of the Z axis comes above the quadratic function curve of the X axis, i.e., the output of the Z axis becomes higher than the output of the X axis. If such an output difference is generated, the detection sensitivity of the sensor drops. In order to decrease this output difference, an amplifier with a different output amplification factor must be provided for each axis. This increases cost.
To solve these shortcomings, Japanese Patent Application Kokai No. 2003-279592 teaches the following structure. The boundary line P1 with the frame section 1 and the boundary line P2 with the mass section 3 are locations where stress concentrates in the beam element 4-1a. The X-axis piezo-resistor 5-1a is disposed at a position contacting the boundary line P1 and the X-axis piezo-resistor 5-1b is disposed at a position contacting the boundary line P2. Also, the distance between the Z-axis piezo-resistors 5-5a and 5-5b is spread as shown in FIG. 4A, or is narrowed (not shown in FIG. 4A), so that the Z-axis piezo-resistors 5-5a and 5-5b are disposed at positions where less influence of the stress-concentrated area is received, and as a result the sensitivity of the Z-axis piezo-resistors 5-5a and 5-5b is decreased and the output difference between the X axis and the Z axis is decreased.